Mineralogy of the Nili Fossae region with OMEGA/Mars Express data: 2. Aqueous alteration of the crust

[1] Nili Fossae is a region with fresh exposures of old material. OMEGA spectral data have revealed a strong diversity of minerals in the Nili Fossae region, including mafic minerals and phyllosilicates. Phyllosilicates are found to be primarily Fe-rich smectites (a type of clay minerals). These phyllosilicates are observed only on the Noachian crust, in four types of locations: Three are without any signatures other than phyllosilicates, on (1) smooth layered deposits, (2) massive exhumed material, and (3) crater ejecta, and one type of location is associated with olivine (4) over an altered mafic unit. This diversity suggests a widespread liquid water activity and a variety of alteration processes. The formation of phyllosilicates underwent considerable reduction, if not cessation, by the time of formation of Syrtis Major lava flows in the Hesperian period. Phyllosilicates are spatially and chronologically disconnected from the fluvial and depositional landforms observed in the Nili Fossae region. Observations suggest that these landforms formed after the extensive formation of phyllosilicates during short periods of time and/or temperatures close to freezing.